I've been running a new Atlas for about a week. Yesterday I recorded 14 false lightning strikes. They occured under bright sunshine and clear skies. I took a fine little radio receiver to my outdoor device and tuned it to the frequency that the lightning detector listens at. I expected to hear noise from the motor. Nope, none detected, even as the fan spun madly. I did hear some of the blurps and bleeps that happen when the internal electronics computes and transmits, but I do not think they matter. The AcuRite engineers almost certainly mute the lightning detector to protect it from hearing that stuff.
Reviewing yesterday's data again this morning I realized that the period of false lightning also coincided with a period of rather low humidity.
At the moment I'm thinking that the spinning fan blade can build up a static charge and an occasional discharge might register as lightning. Think Van de Graaff generator. It's a known issue with fans, and is dealt with by incorporating anti-static materials in the blade plastic. I'll be testing this idea a few different ways, and will report back when I draw a conclusion.